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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>
Devang Patel	3aab76e	2008-03-19 21:56:59 +0000	[diff] [blame]	295	RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>",
				296	false /* Only looks at CFG */,
				297	false /* Analysis Pass */);
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	298	} <i>// end of anonymous namespace</i>
				299	</pre></div>
				300
				301	<p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>,
				302	giving it a command line
Devang Patel	3aab76e	2008-03-19 21:56:59 +0000	[diff] [blame]	303	argument "<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>".
				304	Last two RegisterPass arguments are optional. Their default value is false.
				305	If a pass walks CFG without modifying it then third argument is set to true.
				306	If a pass is an analysis pass, for example dominator tree pass, then true
				307	is supplied as fourth argument. </p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	308
				309	<p>As a whole, the <tt>.cpp</tt> file looks like:</p>
				310
				311	<div class="doc_code"><pre>
				312	<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
				313	<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
				314
				315	<b>using namespace llvm;</b>
				316
				317	<b>namespace</b> {
				318	<b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
				319
				320	static char ID;
				321	Hello() : FunctionPass((intptr_t)&ID) {}
				322
				323	<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
				324	llvm::cerr << "<i>Hello: </i>" << F.getName() << "\n";
				325	<b>return false</b>;
				326	}
				327	};
				328
Devang Patel	8e46f05	2007-07-25 21:05:39 +0000	[diff] [blame]	329	char Hello::ID = 0;
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	330	RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>");
				331	}
				332	</pre></div>
				333
				334	<p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>"
				335	command in the local directory and you should get a new
				336	"<tt>Debug/lib/Hello.so</tt> file. Note that everything in this file is
				337	contained in an anonymous namespace: this reflects the fact that passes are self
				338	contained units that do not need external interfaces (although they can have
				339	them) to be useful.</p>
				340
				341	</div>
				342
				343	<!-- ======================================================================= -->
				344	<div class="doc_subsection">
				345	<a name="running">Running a pass with <tt>opt</tt></a>
				346	</div>
				347
				348	<div class="doc_text">
				349
				350	<p>Now that you have a brand new shiny shared object file, we can use the
				351	<tt>opt</tt> command to run an LLVM program through your pass. Because you
				352	registered your pass with the <tt>RegisterPass</tt> template, you will be able to
				353	use the <tt>opt</tt> tool to access it, once loaded.</p>
				354
				355	<p>To test it, follow the example at the end of the <a
				356	href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to
				357	LLVM. We can now run the bitcode file (<tt>hello.bc</tt>) for the program
				358	through our transformation like this (or course, any bitcode file will
				359	work):</p>
				360
				361	<div class="doc_code"><pre>
				362	$ opt -load ../../../Debug/lib/Hello.so -hello < hello.bc > /dev/null
				363	Hello: __main
				364	Hello: puts
				365	Hello: main
				366	</pre></div>
				367
				368	<p>The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your
				369	pass as a shared object, which makes '<tt>-hello</tt>' a valid command line
				370	argument (which is one reason you need to <a href="#registration">register your
				371	pass</a>). Because the hello pass does not modify the program in any
				372	interesting way, we just throw away the result of <tt>opt</tt> (sending it to
				373	<tt>/dev/null</tt>).</p>
				374
				375	<p>To see what happened to the other string you registered, try running
				376	<tt>opt</tt> with the <tt>--help</tt> option:</p>
				377
				378	<div class="doc_code"><pre>
				379	$ opt -load ../../../Debug/lib/Hello.so --help
				380	OVERVIEW: llvm .bc -> .bc modular optimizer
				381
				382	USAGE: opt [options] <input bitcode>
				383
				384	OPTIONS:
				385	Optimizations available:
				386	...
				387	-funcresolve - Resolve Functions
				388	-gcse - Global Common Subexpression Elimination
				389	-globaldce - Dead Global Elimination
				390	<b>-hello - Hello World Pass</b>
				391	-indvars - Canonicalize Induction Variables
				392	-inline - Function Integration/Inlining
				393	-instcombine - Combine redundant instructions
				394	...
				395	</pre></div>
				396
				397	<p>The pass name get added as the information string for your pass, giving some
				398	documentation to users of <tt>opt</tt>. Now that you have a working pass, you
				399	would go ahead and make it do the cool transformations you want. Once you get
				400	it all working and tested, it may become useful to find out how fast your pass
				401	is. The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command
				402	line option (<tt>--time-passes</tt>) that allows you to get information about
				403	the execution time of your pass along with the other passes you queue up. For
				404	example:</p>
				405
				406	<div class="doc_code"><pre>
				407	$ opt -load ../../../Debug/lib/Hello.so -hello -time-passes < hello.bc > /dev/null
				408	Hello: __main
				409	Hello: puts
				410	Hello: main
				411	===============================================================================
				412	... Pass execution timing report ...
				413	===============================================================================
				414	Total Execution Time: 0.02 seconds (0.0479059 wall clock)
				415
				416	---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name ---
				417	0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bitcode Writer
				418	0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction
				419	0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier
				420	<b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b>
				421	0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL
				422	</pre></div>
				423
				424	<p>As you can see, our implementation above is pretty fast :). The additional
				425	passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify
				426	that the LLVM emitted by your pass is still valid and well formed LLVM, which
				427	hasn't been broken somehow.</p>
				428
				429	<p>Now that you have seen the basics of the mechanics behind passes, we can talk
				430	about some more details of how they work and how to use them.</p>
				431
				432	</div>
				433
				434	<!-- *********************************************************************** -->
				435	<div class="doc_section">
				436	<a name="passtype">Pass classes and requirements</a>
				437	</div>
				438	<!-- *********************************************************************** -->
				439
				440	<div class="doc_text">
				441
				442	<p>One of the first things that you should do when designing a new pass is to
				443	decide what class you should subclass for your pass. The <a
				444	href="#basiccode">Hello World</a> example uses the <tt><a
				445	href="#FunctionPass">FunctionPass</a></tt> class for its implementation, but we
				446	did not discuss why or when this should occur. Here we talk about the classes
				447	available, from the most general to the most specific.</p>
				448
				449	<p>When choosing a superclass for your Pass, you should choose the <b>most
				450	specific</b> class possible, while still being able to meet the requirements
				451	listed. This gives the LLVM Pass Infrastructure information necessary to
				452	optimize how passes are run, so that the resultant compiler isn't unneccesarily
				453	slow.</p>
				454
				455	</div>
				456
				457	<!-- ======================================================================= -->
				458	<div class="doc_subsection">
				459	<a name="ImmutablePass">The <tt>ImmutablePass</tt> class</a>
				460	</div>
				461
				462	<div class="doc_text">
				463
				464	<p>The most plain and boring type of pass is the "<tt><a
				465	href="http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>"
				466	class. This pass type is used for passes that do not have to be run, do not
				467	change state, and never need to be updated. This is not a normal type of
				468	transformation or analysis, but can provide information about the current
				469	compiler configuration.</p>
				470
				471	<p>Although this pass class is very infrequently used, it is important for
				472	providing information about the current target machine being compiled for, and
				473	other static information that can affect the various transformations.</p>
				474
				475	<p><tt>ImmutablePass</tt>es never invalidate other transformations, are never
				476	invalidated, and are never "run".</p>
				477
				478	</div>
				479
				480	<!-- ======================================================================= -->
				481	<div class="doc_subsection">
				482	<a name="ModulePass">The <tt>ModulePass</tt> class</a>
				483	</div>
				484
				485	<div class="doc_text">
				486
				487	<p>The "<tt><a
				488	href="http://llvm.org/doxygen/classllvm_1_1ModulePass.html">ModulePass</a></tt>"
				489	class is the most general of all superclasses that you can use. Deriving from
				490	<tt>ModulePass</tt> indicates that your pass uses the entire program as a unit,
				491	refering to function bodies in no predictable order, or adding and removing
				492	functions. Because nothing is known about the behavior of <tt>ModulePass</tt>
				493	subclasses, no optimization can be done for their execution. A module pass
				494	can use function level passes (e.g. dominators) using getAnalysis interface
Devang Patel	7a9a48f	2008-11-06 19:47:49 +0000	[diff] [blame]	495	<tt> getAnalysis<DominatorTree>(Function)</tt>, if the function pass
				496	does not require any module passes. </p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	497
				498	<p>To write a correct <tt>ModulePass</tt> subclass, derive from
				499	<tt>ModulePass</tt> and overload the <tt>runOnModule</tt> method with the
				500	following signature:</p>
				501
				502	</div>
				503
				504	<!-- _______________________________________________________________________ -->
				505	<div class="doc_subsubsection">
				506	<a name="runOnModule">The <tt>runOnModule</tt> method</a>
				507	</div>
				508
				509	<div class="doc_text">
				510
				511	<div class="doc_code"><pre>
				512	<b>virtual bool</b> runOnModule(Module &M) = 0;
				513	</pre></div>
				514
				515	<p>The <tt>runOnModule</tt> method performs the interesting work of the pass.
				516	It should return true if the module was modified by the transformation and
				517	false otherwise.</p>
				518
				519	</div>
				520
				521	<!-- ======================================================================= -->
				522	<div class="doc_subsection">
				523	<a name="CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
				524	</div>
				525
				526	<div class="doc_text">
				527
				528	<p>The "<tt><a
				529	href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass</a></tt>"
				530	is used by passes that need to traverse the program bottom-up on the call graph
				531	(callees before callers). Deriving from CallGraphSCCPass provides some
				532	mechanics for building and traversing the CallGraph, but also allows the system
				533	to optimize execution of CallGraphSCCPass's. If your pass meets the
				534	requirements outlined below, and doesn't meet the requirements of a <tt><a
				535	href="#FunctionPass">FunctionPass</a></tt> or <tt><a
				536	href="#BasicBlockPass">BasicBlockPass</a></tt>, you should derive from
				537	<tt>CallGraphSCCPass</tt>.</p>
				538
				539	<p><b>TODO</b>: explain briefly what SCC, Tarjan's algo, and B-U mean.</p>
				540
				541	<p>To be explicit, <tt>CallGraphSCCPass</tt> subclasses are:</p>
				542
				543	<ol>
				544
				545	<li>... <em>not allowed</em> to modify any <tt>Function</tt>s that are not in
				546	the current SCC.</li>
				547
				548	<li>... <em>not allowed</em> to inspect any Function's other than those in the
				549	current SCC and the direct callees of the SCC.</li>
				550
				551	<li>... <em>required</em> to preserve the current CallGraph object, updating it
				552	to reflect any changes made to the program.</li>
				553
				554	<li>... <em>not allowed</em> to add or remove SCC's from the current Module,
				555	though they may change the contents of an SCC.</li>
				556
				557	<li>... <em>allowed</em> to add or remove global variables from the current
				558	Module.</li>
				559
				560	<li>... <em>allowed</em> to maintain state across invocations of
				561	<a href="#runOnSCC"><tt>runOnSCC</tt></a> (including global data).</li>
				562	</ol>
				563
				564	<p>Implementing a <tt>CallGraphSCCPass</tt> is slightly tricky in some cases
				565	because it has to handle SCCs with more than one node in it. All of the virtual
				566	methods described below should return true if they modified the program, or
				567	false if they didn't.</p>
				568
				569	</div>
				570
				571	<!-- _______________________________________________________________________ -->
				572	<div class="doc_subsubsection">
				573	<a name="doInitialization_scc">The <tt>doInitialization(CallGraph &)</tt>
				574	method</a>
				575	</div>
				576
				577	<div class="doc_text">
				578
				579	<div class="doc_code"><pre>
				580	<b>virtual bool</b> doInitialization(CallGraph &CG);
				581	</pre></div>
				582
				583	<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
				584	<tt>CallGraphSCCPass</tt>'s are not allowed to do. They can add and remove
				585	functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
				586	is designed to do simple initialization type of stuff that does not depend on
				587	the SCCs being processed. The <tt>doInitialization</tt> method call is not
				588	scheduled to overlap with any other pass executions (thus it should be very
				589	fast).</p>
				590
				591	</div>
				592
				593	<!-- _______________________________________________________________________ -->
				594	<div class="doc_subsubsection">
				595	<a name="runOnSCC">The <tt>runOnSCC</tt> method</a>
				596	</div>
				597
				598	<div class="doc_text">
				599
				600	<div class="doc_code"><pre>
				601	<b>virtual bool</b> runOnSCC(const std::vector<CallGraphNode *> &SCCM) = 0;
				602	</pre></div>
				603
				604	<p>The <tt>runOnSCC</tt> method performs the interesting work of the pass, and
				605	should return true if the module was modified by the transformation, false
				606	otherwise.</p>
				607
				608	</div>
				609
				610	<!-- _______________________________________________________________________ -->
				611	<div class="doc_subsubsection">
				612	<a name="doFinalization_scc">The <tt>doFinalization(CallGraph
				613	&)</tt> method</a>
				614	</div>
				615
				616	<div class="doc_text">
				617
				618	<div class="doc_code"><pre>
				619	<b>virtual bool</b> doFinalization(CallGraph &CG);
				620	</pre></div>
				621
				622	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				623	called when the pass framework has finished calling <a
				624	href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
				625	program being compiled.</p>
				626
				627	</div>
				628
				629	<!-- ======================================================================= -->
				630	<div class="doc_subsection">
				631	<a name="FunctionPass">The <tt>FunctionPass</tt> class</a>
				632	</div>
				633
				634	<div class="doc_text">
				635
				636	<p>In contrast to <tt>ModulePass</tt> subclasses, <tt><a
				637	href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt>
				638	subclasses do have a predictable, local behavior that can be expected by the
				639	system. All <tt>FunctionPass</tt> execute on each function in the program
				640	independent of all of the other functions in the program.
				641	<tt>FunctionPass</tt>'s do not require that they are executed in a particular
				642	order, and <tt>FunctionPass</tt>'s do not modify external functions.</p>
				643
				644	<p>To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:</p>
				645
				646	<ol>
				647	<li>Modify a Function other than the one currently being processed.</li>
				648	<li>Add or remove Function's from the current Module.</li>
				649	<li>Add or remove global variables from the current Module.</li>
				650	<li>Maintain state across invocations of
				651	<a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)</li>
				652	</ol>
				653
				654	<p>Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a
				655	href="#basiccode">Hello World</a> pass for example). <tt>FunctionPass</tt>'s
				656	may overload three virtual methods to do their work. All of these methods
				657	should return true if they modified the program, or false if they didn't.</p>
				658
				659	</div>
				660
				661	<!-- _______________________________________________________________________ -->
				662	<div class="doc_subsubsection">
				663	<a name="doInitialization_mod">The <tt>doInitialization(Module &)</tt>
				664	method</a>
				665	</div>
				666
				667	<div class="doc_text">
				668
				669	<div class="doc_code"><pre>
				670	<b>virtual bool</b> doInitialization(Module &M);
				671	</pre></div>
				672
				673	<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
				674	<tt>FunctionPass</tt>'s are not allowed to do. They can add and remove
				675	functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
				676	is designed to do simple initialization type of stuff that does not depend on
				677	the functions being processed. The <tt>doInitialization</tt> method call is not
				678	scheduled to overlap with any other pass executions (thus it should be very
				679	fast).</p>
				680
				681	<p>A good example of how this method should be used is the <a
				682	href="http://llvm.org/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a>
				683	pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into
				684	platform dependent <tt>malloc()</tt> and <tt>free()</tt> function calls. It
				685	uses the <tt>doInitialization</tt> method to get a reference to the malloc and
				686	free functions that it needs, adding prototypes to the module if necessary.</p>
				687
				688	</div>
				689
				690	<!-- _______________________________________________________________________ -->
				691	<div class="doc_subsubsection">
				692	<a name="runOnFunction">The <tt>runOnFunction</tt> method</a>
				693	</div>
				694
				695	<div class="doc_text">
				696
				697	<div class="doc_code"><pre>
				698	<b>virtual bool</b> runOnFunction(Function &F) = 0;
				699	</pre></div><p>
				700
				701	<p>The <tt>runOnFunction</tt> method must be implemented by your subclass to do
				702	the transformation or analysis work of your pass. As usual, a true value should
				703	be returned if the function is modified.</p>
				704
				705	</div>
				706
				707	<!-- _______________________________________________________________________ -->
				708	<div class="doc_subsubsection">
				709	<a name="doFinalization_mod">The <tt>doFinalization(Module
				710	&)</tt> method</a>
				711	</div>
				712
				713	<div class="doc_text">
				714
				715	<div class="doc_code"><pre>
				716	<b>virtual bool</b> doFinalization(Module &M);
				717	</pre></div>
				718
				719	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				720	called when the pass framework has finished calling <a
				721	href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
				722	program being compiled.</p>
				723
				724	</div>
				725
				726	<!-- ======================================================================= -->
				727	<div class="doc_subsection">
				728	<a name="LoopPass">The <tt>LoopPass</tt> class </a>
				729	</div>
				730
				731	<div class="doc_text">
				732
				733	<p> All <tt>LoopPass</tt> execute on each loop in the function independent of
				734	all of the other loops in the function. <tt>LoopPass</tt> processes loops in
				735	loop nest order such that outer most loop is processed last. </p>
				736
				737	<p> <tt>LoopPass</tt> subclasses are allowed to update loop nest using
				738	<tt>LPPassManager</tt> interface. Implementing a loop pass is usually
				739	straightforward. <tt>Looppass</tt>'s may overload three virtual methods to
				740	do their work. All these methods should return true if they modified the
				741	program, or false if they didn't. </p>
				742	</div>
				743
				744	<!-- _______________________________________________________________________ -->
				745	<div class="doc_subsubsection">
				746	<a name="doInitialization_loop">The <tt>doInitialization(Loop *,
				747	LPPassManager &)</tt>
				748	method</a>
				749	</div>
				750
				751	<div class="doc_text">
				752
				753	<div class="doc_code"><pre>
				754	<b>virtual bool</b> doInitialization(Loop *, LPPassManager &LPM);
				755	</pre></div>
				756
				757	<p>The <tt>doInitialization</tt> method is designed to do simple initialization
				758	type of stuff that does not depend on the functions being processed. The
				759	<tt>doInitialization</tt> method call is not scheduled to overlap with any
				760	other pass executions (thus it should be very fast). LPPassManager
				761	interface should be used to access Function or Module level analysis
				762	information.</p>
				763
				764	</div>
				765
				766
				767	<!-- _______________________________________________________________________ -->
				768	<div class="doc_subsubsection">
				769	<a name="runOnLoop">The <tt>runOnLoop</tt> method</a>
				770	</div>
				771
				772	<div class="doc_text">
				773
				774	<div class="doc_code"><pre>
				775	<b>virtual bool</b> runOnLoop(Loop *, LPPassManager &LPM) = 0;
				776	</pre></div><p>
				777
				778	<p>The <tt>runOnLoop</tt> method must be implemented by your subclass to do
				779	the transformation or analysis work of your pass. As usual, a true value should
				780	be returned if the function is modified. <tt>LPPassManager</tt> interface
				781	should be used to update loop nest.</p>
				782
				783	</div>
				784
				785	<!-- _______________________________________________________________________ -->
				786	<div class="doc_subsubsection">
				787	<a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a>
				788	</div>
				789
				790	<div class="doc_text">
				791
				792	<div class="doc_code"><pre>
				793	<b>virtual bool</b> doFinalization();
				794	</pre></div>
				795
				796	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				797	called when the pass framework has finished calling <a
				798	href="#runOnLoop"><tt>runOnLoop</tt></a> for every loop in the
				799	program being compiled. </p>
				800
				801	</div>
				802
				803
				804
				805	<!-- ======================================================================= -->
				806	<div class="doc_subsection">
				807	<a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
				808	</div>
				809
				810	<div class="doc_text">
				811
				812	<p><tt>BasicBlockPass</tt>'s are just like <a
				813	href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit
				814	their scope of inspection and modification to a single basic block at a time.
				815	As such, they are <b>not</b> allowed to do any of the following:</p>
				816
				817	<ol>
				818	<li>Modify or inspect any basic blocks outside of the current one</li>
				819	<li>Maintain state across invocations of
				820	<a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a></li>
				821	<li>Modify the control flow graph (by altering terminator instructions)</li>
				822	<li>Any of the things forbidden for
				823	<a href="#FunctionPass"><tt>FunctionPass</tt></a>es.</li>
				824	</ol>
				825
				826	<p><tt>BasicBlockPass</tt>es are useful for traditional local and "peephole"
				827	optimizations. They may override the same <a
				828	href="#doInitialization_mod"><tt>doInitialization(Module &)</tt></a> and <a
				829	href="#doFinalization_mod"><tt>doFinalization(Module &)</tt></a> methods that <a
				830	href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:</p>
				831
				832	</div>
				833
				834	<!-- _______________________________________________________________________ -->
				835	<div class="doc_subsubsection">
				836	<a name="doInitialization_fn">The <tt>doInitialization(Function
				837	&)</tt> method</a>
				838	</div>
				839
				840	<div class="doc_text">
				841
				842	<div class="doc_code"><pre>
				843	<b>virtual bool</b> doInitialization(Function &F);
				844	</pre></div>
				845
				846	<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
				847	<tt>BasicBlockPass</tt>'s are not allowed to do, but that
				848	<tt>FunctionPass</tt>'s can. The <tt>doInitialization</tt> method is designed
				849	to do simple initialization that does not depend on the
				850	BasicBlocks being processed. The <tt>doInitialization</tt> method call is not
				851	scheduled to overlap with any other pass executions (thus it should be very
				852	fast).</p>
				853
				854	</div>
				855
				856	<!-- _______________________________________________________________________ -->
				857	<div class="doc_subsubsection">
				858	<a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
				859	</div>
				860
				861	<div class="doc_text">
				862
				863	<div class="doc_code"><pre>
				864	<b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0;
				865	</pre></div>
				866
				867	<p>Override this function to do the work of the <tt>BasicBlockPass</tt>. This
				868	function is not allowed to inspect or modify basic blocks other than the
				869	parameter, and are not allowed to modify the CFG. A true value must be returned
				870	if the basic block is modified.</p>
				871
				872	</div>
				873
				874	<!-- _______________________________________________________________________ -->
				875	<div class="doc_subsubsection">
				876	<a name="doFinalization_fn">The <tt>doFinalization(Function &)</tt>
				877	method</a>
				878	</div>
				879
				880	<div class="doc_text">
				881
				882	<div class="doc_code"><pre>
				883	<b>virtual bool</b> doFinalization(Function &F);
				884	</pre></div>
				885
				886	<p>The <tt>doFinalization</tt> method is an infrequently used method that is
				887	called when the pass framework has finished calling <a
				888	href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the
				889	program being compiled. This can be used to perform per-function
				890	finalization.</p>
				891
				892	</div>
				893
				894	<!-- ======================================================================= -->
				895	<div class="doc_subsection">
				896	<a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a>
				897	</div>
				898
				899	<div class="doc_text">
				900
				901	<p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that
				902	executes on the machine-dependent representation of each LLVM function in the
				903	program. A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
				904	the restrictions that apply to a <tt>FunctionPass</tt> also apply to it.
				905	<tt>MachineFunctionPass</tt>es also have additional restrictions. In particular,
				906	<tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p>
				907
				908	<ol>
				909	<li>Modify any LLVM Instructions, BasicBlocks or Functions.</li>
				910	<li>Modify a MachineFunction other than the one currently being processed.</li>
				911	<li>Add or remove MachineFunctions from the current Module.</li>
				912	<li>Add or remove global variables from the current Module.</li>
				913	<li>Maintain state across invocations of <a
				914	href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global
				915	data)</li>
				916	</ol>
				917
				918	</div>
				919
				920	<!-- _______________________________________________________________________ -->
				921	<div class="doc_subsubsection">
				922	<a name="runOnMachineFunction">The <tt>runOnMachineFunction(MachineFunction
				923	&MF)</tt> method</a>
				924	</div>
				925
				926	<div class="doc_text">
				927
				928	<div class="doc_code"><pre>
				929	<b>virtual bool</b> runOnMachineFunction(MachineFunction &MF) = 0;
				930	</pre></div>
				931
				932	<p><tt>runOnMachineFunction</tt> can be considered the main entry point of a
				933	<tt>MachineFunctionPass</tt>; that is, you should override this method to do the
				934	work of your <tt>MachineFunctionPass</tt>.</p>
				935
				936	<p>The <tt>runOnMachineFunction</tt> method is called on every
				937	<tt>MachineFunction</tt> in a <tt>Module</tt>, so that the
				938	<tt>MachineFunctionPass</tt> may perform optimizations on the machine-dependent
				939	representation of the function. If you want to get at the LLVM <tt>Function</tt>
				940	for the <tt>MachineFunction</tt> you're working on, use
				941	<tt>MachineFunction</tt>'s <tt>getFunction()</tt> accessor method -- but
				942	remember, you may not modify the LLVM <tt>Function</tt> or its contents from a
				943	<tt>MachineFunctionPass</tt>.</p>
				944
				945	</div>
				946
				947	<!-- *********************************************************************** -->
				948	<div class="doc_section">
				949	<a name="registration">Pass registration</a>
				950	</div>
				951	<!-- *********************************************************************** -->
				952
				953	<div class="doc_text">
				954
				955	<p>In the <a href="#basiccode">Hello World</a> example pass we illustrated how
				956	pass registration works, and discussed some of the reasons that it is used and
				957	what it does. Here we discuss how and why passes are registered.</p>
				958
				959	<p>As we saw above, passes are registered with the <b><tt>RegisterPass</tt></b>
				960	template, which requires you to pass at least two
				961	parameters. The first parameter is the name of the pass that is to be used on
				962	the command line to specify that the pass should be added to a program (for
				963	example, with <tt>opt</tt> or <tt>bugpoint</tt>). The second argument is the
				964	name of the pass, which is to be used for the <tt>--help</tt> output of
				965	programs, as
				966	well as for debug output generated by the <tt>--debug-pass</tt> option.</p>
				967
				968	<p>If you want your pass to be easily dumpable, you should
				969	implement the virtual <tt>print</tt> method:</p>
				970
				971	</div>
				972
				973	<!-- _______________________________________________________________________ -->
				974	<div class="doc_subsubsection">
				975	<a name="print">The <tt>print</tt> method</a>
				976	</div>
				977
				978	<div class="doc_text">
				979
				980	<div class="doc_code"><pre>
Edwin Török	72a8fd2	2008-10-28 17:29:23 +0000	[diff] [blame]	981	<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]	982	</pre></div>
				983
				984	<p>The <tt>print</tt> method must be implemented by "analyses" in order to print
				985	a human readable version of the analysis results. This is useful for debugging
				986	an analysis itself, as well as for other people to figure out how an analysis
				987	works. Use the <tt>opt -analyze</tt> argument to invoke this method.</p>
				988
				989	<p>The <tt>llvm::OStream</tt> parameter specifies the stream to write the results on,
				990	and the <tt>Module</tt> parameter gives a pointer to the top level module of the
				991	program that has been analyzed. Note however that this pointer may be null in
				992	certain circumstances (such as calling the <tt>Pass::dump()</tt> from a
				993	debugger), so it should only be used to enhance debug output, it should not be
				994	depended on.</p>
				995
				996	</div>
				997
				998	<!-- *********************************************************************** -->
				999	<div class="doc_section">
				1000	<a name="interaction">Specifying interactions between passes</a>
				1001	</div>
				1002	<!-- *********************************************************************** -->
				1003
				1004	<div class="doc_text">
				1005
John Criswell	a99e43f	2007-12-03 19:34:25 +0000	[diff] [blame]	1006	<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]	1007	that passes interact with each other correctly. Because <tt>PassManager</tt>
				1008	tries to <a href="#passmanager">optimize the execution of passes</a> it must
				1009	know how the passes interact with each other and what dependencies exist between
				1010	the various passes. To track this, each pass can declare the set of passes that
				1011	are required to be executed before the current pass, and the passes which are
				1012	invalidated by the current pass.</p>
				1013
				1014	<p>Typically this functionality is used to require that analysis results are
				1015	computed before your pass is run. Running arbitrary transformation passes can
				1016	invalidate the computed analysis results, which is what the invalidation set
				1017	specifies. If a pass does not implement the <tt><a
				1018	href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not
				1019	having any prerequisite passes, and invalidating <b>all</b> other passes.</p>
				1020
				1021	</div>
				1022
				1023	<!-- _______________________________________________________________________ -->
				1024	<div class="doc_subsubsection">
				1025	<a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a>
				1026	</div>
				1027
				1028	<div class="doc_text">
				1029
				1030	<div class="doc_code"><pre>
				1031	<b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>;
				1032	</pre></div>
				1033
				1034	<p>By implementing the <tt>getAnalysisUsage</tt> method, the required and
				1035	invalidated sets may be specified for your transformation. The implementation
				1036	should fill in the <tt><a
				1037	href="http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
				1038	object with information about which passes are required and not invalidated. To
				1039	do this, a pass may call any of the following methods on the AnalysisUsage
				1040	object:</p>
				1041	</div>
				1042
				1043	<!-- _______________________________________________________________________ -->
				1044	<div class="doc_subsubsection">
				1045	<a name="AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a>
				1046	</div>
				1047
				1048	<div class="doc_text">
				1049	<p>
				1050	If your pass requires a previous pass to be executed (an analysis for example),
				1051	it can use one of these methods to arrange for it to be run before your pass.
				1052	LLVM has many different types of analyses and passes that can be required,
				1053	spanning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>.
				1054	Requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will
				1055	be no critical edges in the CFG when your pass has been run.
				1056	</p>
				1057
				1058	<p>
				1059	Some analyses chain to other analyses to do their job. For example, an <a
				1060	href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a
				1061	href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes. In
				1062	cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be
				1063	used instead of the <tt>addRequired</tt> method. This informs the PassManager
				1064	that the transitively required pass should be alive as long as the requiring
				1065	pass is.
				1066	</p>
				1067	</div>
				1068
				1069	<!-- _______________________________________________________________________ -->
				1070	<div class="doc_subsubsection">
				1071	<a name="AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a>
				1072	</div>
				1073
				1074	<div class="doc_text">
				1075	<p>
				1076	One of the jobs of the PassManager is to optimize how and when analyses are run.
				1077	In particular, it attempts to avoid recomputing data unless it needs to. For
				1078	this reason, passes are allowed to declare that they preserve (i.e., they don't
				1079	invalidate) an existing analysis if it's available. For example, a simple
				1080	constant folding pass would not modify the CFG, so it can't possibly affect the
				1081	results of dominator analysis. By default, all passes are assumed to invalidate
				1082	all others.
				1083	</p>
				1084
				1085	<p>
				1086	The <tt>AnalysisUsage</tt> class provides several methods which are useful in
				1087	certain circumstances that are related to <tt>addPreserved</tt>. In particular,
				1088	the <tt>setPreservesAll</tt> method can be called to indicate that the pass does
				1089	not modify the LLVM program at all (which is true for analyses), and the
				1090	<tt>setPreservesCFG</tt> method can be used by transformations that change
				1091	instructions in the program but do not modify the CFG or terminator instructions
				1092	(note that this property is implicitly set for <a
				1093	href="#BasicBlockPass">BasicBlockPass</a>'s).
				1094	</p>
				1095
				1096	<p>
				1097	<tt>addPreserved</tt> is particularly useful for transformations like
				1098	<tt>BreakCriticalEdges</tt>. This pass knows how to update a small set of loop
				1099	and dominator related analyses if they exist, so it can preserve them, despite
				1100	the fact that it hacks on the CFG.
				1101	</p>
				1102	</div>
				1103
				1104	<!-- _______________________________________________________________________ -->
				1105	<div class="doc_subsubsection">
				1106	<a name="AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a>
				1107	</div>
				1108
				1109	<div class="doc_text">
				1110
				1111	<div class="doc_code"><pre>
				1112	<i>// This is an example implementation from an analysis, which does not modify
				1113	// the program at all, yet has a prerequisite.</i>
				1114	<b>void</b> <a href="http://llvm.org/doxygen/classllvm_1_1PostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
				1115	AU.setPreservesAll();
				1116	AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1PostDominatorTree.html">PostDominatorTree</a>>();
				1117	}
				1118	</pre></div>
				1119
				1120	<p>and:</p>
				1121
				1122	<div class="doc_code"><pre>
				1123	<i>// This example modifies the program, but does not modify the CFG</i>
				1124	<b>void</b> <a href="http://llvm.org/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
				1125	AU.setPreservesCFG();
				1126	AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>>();
				1127	}
				1128	</pre></div>
				1129
				1130	</div>
				1131
				1132	<!-- _______________________________________________________________________ -->
				1133	<div class="doc_subsubsection">
				1134	<a name="getAnalysis">The <tt>getAnalysis<></tt> and <tt>getAnalysisToUpdate<></tt> methods</a>
				1135	</div>
				1136
				1137	<div class="doc_text">
				1138
				1139	<p>The <tt>Pass::getAnalysis<></tt> method is automatically inherited by
				1140	your class, providing you with access to the passes that you declared that you
				1141	required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a>
				1142	method. It takes a single template argument that specifies which pass class you
				1143	want, and returns a reference to that pass. For example:</p>
				1144
				1145	<div class="doc_code"><pre>
				1146	bool LICM::runOnFunction(Function &F) {
				1147	LoopInfo &LI = getAnalysis<LoopInfo>();
				1148	...
				1149	}
				1150	</pre></div>
				1151
				1152	<p>This method call returns a reference to the pass desired. You may get a
				1153	runtime assertion failure if you attempt to get an analysis that you did not
				1154	declare as required in your <a
				1155	href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation. This
				1156	method can be called by your <tt>run*</tt> method implementation, or by any
				1157	other local method invoked by your <tt>run*</tt> method.
				1158
				1159	A module level pass can use function level analysis info using this interface.
				1160	For example:</p>
				1161
				1162	<div class="doc_code"><pre>
				1163	bool ModuleLevelPass::runOnModule(Module &M) {
				1164	...
				1165	DominatorTree &DT = getAnalysis<DominatorTree>(Func);
				1166	...
				1167	}
				1168	</pre></div>
				1169
				1170	<p>In above example, runOnFunction for DominatorTree is called by pass manager
				1171	before returning a reference to the desired pass.</p>
				1172
				1173	<p>
				1174	If your pass is capable of updating analyses if they exist (e.g.,
				1175	<tt>BreakCriticalEdges</tt>, as described above), you can use the
				1176	<tt>getAnalysisToUpdate</tt> method, which returns a pointer to the analysis if
				1177	it is active. For example:</p>
				1178
				1179	<div class="doc_code"><pre>
				1180	...
				1181	if (DominatorSet *DS = getAnalysisToUpdate<DominatorSet>()) {
				1182	<i>// A DominatorSet is active. This code will update it.</i>
				1183	}
				1184	...
				1185	</pre></div>
				1186
				1187	</div>
				1188
				1189	<!-- *********************************************************************** -->
				1190	<div class="doc_section">
				1191	<a name="analysisgroup">Implementing Analysis Groups</a>
				1192	</div>
				1193	<!-- *********************************************************************** -->
				1194
				1195	<div class="doc_text">
				1196
Chris Lattner	942c395	2007-11-16 05:32:05 +0000	[diff] [blame]	1197	<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]	1198	used, and how they are required from other passes, it's time to get a little bit
				1199	fancier. All of the pass relationships that we have seen so far are very
				1200	simple: one pass depends on one other specific pass to be run before it can run.
				1201	For many applications, this is great, for others, more flexibility is
				1202	required.</p>
				1203
				1204	<p>In particular, some analyses are defined such that there is a single simple
				1205	interface to the analysis results, but multiple ways of calculating them.
				1206	Consider alias analysis for example. The most trivial alias analysis returns
				1207	"may alias" for any alias query. The most sophisticated analysis a
				1208	flow-sensitive, context-sensitive interprocedural analysis that can take a
				1209	significant amount of time to execute (and obviously, there is a lot of room
				1210	between these two extremes for other implementations). To cleanly support
				1211	situations like this, the LLVM Pass Infrastructure supports the notion of
				1212	Analysis Groups.</p>
				1213
				1214	</div>
				1215
				1216	<!-- _______________________________________________________________________ -->
				1217	<div class="doc_subsubsection">
				1218	<a name="agconcepts">Analysis Group Concepts</a>
				1219	</div>
				1220
				1221	<div class="doc_text">
				1222
				1223	<p>An Analysis Group is a single simple interface that may be implemented by
				1224	multiple different passes. Analysis Groups can be given human readable names
				1225	just like passes, but unlike passes, they need not derive from the <tt>Pass</tt>
				1226	class. An analysis group may have one or more implementations, one of which is
				1227	the "default" implementation.</p>
				1228
				1229	<p>Analysis groups are used by client passes just like other passes are: the
				1230	<tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods.
				1231	In order to resolve this requirement, the <a href="#passmanager">PassManager</a>
				1232	scans the available passes to see if any implementations of the analysis group
				1233	are available. If none is available, the default implementation is created for
				1234	the pass to use. All standard rules for <A href="#interaction">interaction
				1235	between passes</a> still apply.</p>
				1236
				1237	<p>Although <a href="#registration">Pass Registration</a> is optional for normal
				1238	passes, all analysis group implementations must be registered, and must use the
				1239	<A href="#registerag"><tt>RegisterAnalysisGroup</tt></a> template to join the
				1240	implementation pool. Also, a default implementation of the interface
				1241	<b>must</b> be registered with <A
				1242	href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p>
				1243
				1244	<p>As a concrete example of an Analysis Group in action, consider the <a
				1245	href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
				1246	analysis group. The default implementation of the alias analysis interface (the
				1247	<tt><a
				1248	href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt>
				1249	pass) just does a few simple checks that don't require significant analysis to
				1250	compute (such as: two different globals can never alias each other, etc).
				1251	Passes that use the <tt><a
				1252	href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
				1253	interface (for example the <tt><a
				1254	href="http://llvm.org/doxygen/structGCSE.html">gcse</a></tt> pass), do
				1255	not care which implementation of alias analysis is actually provided, they just
				1256	use the designated interface.</p>
				1257
				1258	<p>From the user's perspective, commands work just like normal. Issuing the
				1259	command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be
				1260	instantiated and added to the pass sequence. Issuing the command '<tt>opt
				1261	-somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the
				1262	<tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a
				1263	hypothetical example) instead.</p>
				1264
				1265	</div>
				1266
				1267	<!-- _______________________________________________________________________ -->
				1268	<div class="doc_subsubsection">
				1269	<a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a>
				1270	</div>
				1271
				1272	<div class="doc_text">
				1273
				1274	<p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis
				1275	group itself as well as add pass implementations to the analysis group. First,
				1276	an analysis should be registered, with a human readable name provided for it.
				1277	Unlike registration of passes, there is no command line argument to be specified
				1278	for the Analysis Group Interface itself, because it is "abstract":</p>
				1279
				1280	<div class="doc_code"><pre>
				1281	<b>static</b> RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> A("<i>Alias Analysis</i>");
				1282	</pre></div>
				1283
				1284	<p>Once the analysis is registered, passes can declare that they are valid
				1285	implementations of the interface by using the following code:</p>
				1286
				1287	<div class="doc_code"><pre>
				1288	<b>namespace</b> {
				1289	//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
				1290	RegisterPass<FancyAA>
				1291	B("<i>somefancyaa</i>", "<i>A more complex alias analysis implementation</i>");
				1292
				1293	//<i> Declare that we implement the AliasAnalysis interface</i>
				1294	RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> C(B);
				1295	}
				1296	</pre></div>
				1297
				1298	<p>This just shows a class <tt>FancyAA</tt> that is registered normally, then
				1299	uses the <tt>RegisterAnalysisGroup</tt> template to "join" the <tt><a
				1300	href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
				1301	analysis group. Every implementation of an analysis group should join using
				1302	this template. A single pass may join multiple different analysis groups with
				1303	no problem.</p>
				1304
				1305	<div class="doc_code"><pre>
				1306	<b>namespace</b> {
				1307	//<i> Analysis Group implementations <b>must</b> be registered normally...</i>
				1308	RegisterPass<<a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>>
				1309	D("<i>basicaa</i>", "<i>Basic Alias Analysis (default AA impl)</i>");
				1310
				1311	//<i> Declare that we implement the AliasAnalysis interface</i>
				1312	RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, <b>true</b>> E(D);
				1313	}
				1314	</pre></div>
				1315
				1316	<p>Here we show how the default implementation is specified (using the extra
				1317	argument to the <tt>RegisterAnalysisGroup</tt> template). There must be exactly
				1318	one default implementation available at all times for an Analysis Group to be
				1319	used. Only default implementation can derive from <tt>ImmutablePass</tt>.
				1320	Here we declare that the
				1321	<tt><a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt>
				1322	pass is the default implementation for the interface.</p>
				1323
				1324	</div>
				1325
				1326	<!-- *********************************************************************** -->
				1327	<div class="doc_section">
				1328	<a name="passStatistics">Pass Statistics</a>
				1329	</div>
				1330	<!-- *********************************************************************** -->
				1331
				1332	<div class="doc_text">
				1333	<p>The <a
				1334	href="http://llvm.org/doxygen/Statistic_8h-source.html"><tt>Statistic</tt></a>
				1335	class is designed to be an easy way to expose various success
				1336	metrics from passes. These statistics are printed at the end of a
				1337	run, when the -stats command line option is enabled on the command
				1338	line. See the <a href="http://llvm.org/docs/ProgrammersManual.html#Statistic">Statistics section</a> in the Programmer's Manual for details.
				1339
				1340	</div>
				1341
				1342
				1343	<!-- *********************************************************************** -->
				1344	<div class="doc_section">
				1345	<a name="passmanager">What PassManager does</a>
				1346	</div>
				1347	<!-- *********************************************************************** -->
				1348
				1349	<div class="doc_text">
				1350
				1351	<p>The <a
				1352	href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
				1353	<a
				1354	href="http://llvm.org/doxygen/classllvm_1_1PassManager.html">class</a>
				1355	takes a list of passes, ensures their <a href="#interaction">prerequisites</a>
				1356	are set up correctly, and then schedules passes to run efficiently. All of the
				1357	LLVM tools that run passes use the <tt>PassManager</tt> for execution of these
				1358	passes.</p>
				1359
				1360	<p>The <tt>PassManager</tt> does two main things to try to reduce the execution
				1361	time of a series of passes:</p>
				1362
				1363	<ol>
				1364	<li><b>Share analysis results</b> - The PassManager attempts to avoid
				1365	recomputing analysis results as much as possible. This means keeping track of
				1366	which analyses are available already, which analyses get invalidated, and which
				1367	analyses are needed to be run for a pass. An important part of work is that the
				1368	<tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing
				1369	it to <a href="#releaseMemory">free memory</a> allocated to holding analysis
				1370	results as soon as they are no longer needed.</li>
				1371
				1372	<li><b>Pipeline the execution of passes on the program</b> - The
				1373	<tt>PassManager</tt> attempts to get better cache and memory usage behavior out
				1374	of a series of passes by pipelining the passes together. This means that, given
				1375	a series of consequtive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it
				1376	will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on
				1377	the first function, then all of the <a
				1378	href="#FunctionPass"><tt>FunctionPass</tt></a>es on the second function,
				1379	etc... until the entire program has been run through the passes.
				1380
				1381	<p>This improves the cache behavior of the compiler, because it is only touching
				1382	the LLVM program representation for a single function at a time, instead of
				1383	traversing the entire program. It reduces the memory consumption of compiler,
				1384	because, for example, only one <a
				1385	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]	1386	needs to be calculated at a time. This also makes it possible to implement
				1387	some <a
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1388	href="#SMP">interesting enhancements</a> in the future.</p></li>
				1389
				1390	</ol>
				1391
				1392	<p>The effectiveness of the <tt>PassManager</tt> is influenced directly by how
				1393	much information it has about the behaviors of the passes it is scheduling. For
				1394	example, the "preserved" set is intentionally conservative in the face of an
				1395	unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method.
				1396	Not implementing when it should be implemented will have the effect of not
				1397	allowing any analysis results to live across the execution of your pass.</p>
				1398
				1399	<p>The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line
				1400	options that is useful for debugging pass execution, seeing how things work, and
				1401	diagnosing when you should be preserving more analyses than you currently are
				1402	(To get information about all of the variants of the <tt>--debug-pass</tt>
				1403	option, just type '<tt>opt --help-hidden</tt>').</p>
				1404
				1405	<p>By using the <tt>--debug-pass=Structure</tt> option, for example, we can see
				1406	how our <a href="#basiccode">Hello World</a> pass interacts with other passes.
				1407	Lets try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:</p>
				1408
				1409	<div class="doc_code"><pre>
				1410	$ opt -load ../../../Debug/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null
				1411	Module Pass Manager
				1412	Function Pass Manager
				1413	Dominator Set Construction
				1414	Immediate Dominators Construction
				1415	Global Common Subexpression Elimination
				1416	-- Immediate Dominators Construction
				1417	-- Global Common Subexpression Elimination
				1418	Natural Loop Construction
				1419	Loop Invariant Code Motion
				1420	-- Natural Loop Construction
				1421	-- Loop Invariant Code Motion
				1422	Module Verifier
				1423	-- Dominator Set Construction
				1424	-- Module Verifier
				1425	Bitcode Writer
				1426	--Bitcode Writer
				1427	</pre></div>
				1428
				1429	<p>This output shows us when passes are constructed and when the analysis
				1430	results are known to be dead (prefixed with '<tt>--</tt>'). Here we see that
				1431	GCSE uses dominator and immediate dominator information to do its job. The LICM
				1432	pass uses natural loop information, which uses dominator sets, but not immediate
				1433	dominators. Because immediate dominators are no longer useful after the GCSE
				1434	pass, it is immediately destroyed. The dominator sets are then reused to
				1435	compute natural loop information, which is then used by the LICM pass.</p>
				1436
				1437	<p>After the LICM pass, the module verifier runs (which is automatically added
				1438	by the '<tt>opt</tt>' tool), which uses the dominator set to check that the
				1439	resultant LLVM code is well formed. After it finishes, the dominator set
				1440	information is destroyed, after being computed once, and shared by three
				1441	passes.</p>
				1442
				1443	<p>Lets see how this changes when we run the <a href="#basiccode">Hello
				1444	World</a> pass in between the two passes:</p>
				1445
				1446	<div class="doc_code"><pre>
				1447	$ opt -load ../../../Debug/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
				1448	Module Pass Manager
				1449	Function Pass Manager
				1450	Dominator Set Construction
				1451	Immediate Dominators Construction
				1452	Global Common Subexpression Elimination
				1453	<b>-- Dominator Set Construction</b>
				1454	-- Immediate Dominators Construction
				1455	-- Global Common Subexpression Elimination
				1456	<b> Hello World Pass
				1457	-- Hello World Pass
				1458	Dominator Set Construction</b>
				1459	Natural Loop Construction
				1460	Loop Invariant Code Motion
				1461	-- Natural Loop Construction
				1462	-- Loop Invariant Code Motion
				1463	Module Verifier
				1464	-- Dominator Set Construction
				1465	-- Module Verifier
				1466	Bitcode Writer
				1467	--Bitcode Writer
				1468	Hello: __main
				1469	Hello: puts
				1470	Hello: main
				1471	</pre></div>
				1472
				1473	<p>Here we see that the <a href="#basiccode">Hello World</a> pass has killed the
				1474	Dominator Set pass, even though it doesn't modify the code at all! To fix this,
				1475	we need to add the following <a
				1476	href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:</p>
				1477
				1478	<div class="doc_code"><pre>
				1479	<i>// We don't modify the program, so we preserve all analyses</i>
				1480	<b>virtual void</b> getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
				1481	AU.setPreservesAll();
				1482	}
				1483	</pre></div>
				1484
				1485	<p>Now when we run our pass, we get this output:</p>
				1486
				1487	<div class="doc_code"><pre>
				1488	$ opt -load ../../../Debug/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
				1489	Pass Arguments: -gcse -hello -licm
				1490	Module Pass Manager
				1491	Function Pass Manager
				1492	Dominator Set Construction
				1493	Immediate Dominators Construction
				1494	Global Common Subexpression Elimination
				1495	-- Immediate Dominators Construction
				1496	-- Global Common Subexpression Elimination
				1497	Hello World Pass
				1498	-- Hello World Pass
				1499	Natural Loop Construction
				1500	Loop Invariant Code Motion
				1501	-- Loop Invariant Code Motion
				1502	-- Natural Loop Construction
				1503	Module Verifier
				1504	-- Dominator Set Construction
				1505	-- Module Verifier
				1506	Bitcode Writer
				1507	--Bitcode Writer
				1508	Hello: __main
				1509	Hello: puts
				1510	Hello: main
				1511	</pre></div>
				1512
				1513	<p>Which shows that we don't accidentally invalidate dominator information
				1514	anymore, and therefore do not have to compute it twice.</p>
				1515
				1516	</div>
				1517
				1518	<!-- _______________________________________________________________________ -->
				1519	<div class="doc_subsubsection">
				1520	<a name="releaseMemory">The <tt>releaseMemory</tt> method</a>
				1521	</div>
				1522
				1523	<div class="doc_text">
				1524
				1525	<div class="doc_code"><pre>
				1526	<b>virtual void</b> releaseMemory();
				1527	</pre></div>
				1528
				1529	<p>The <tt>PassManager</tt> automatically determines when to compute analysis
				1530	results, and how long to keep them around for. Because the lifetime of the pass
				1531	object itself is effectively the entire duration of the compilation process, we
				1532	need some way to free analysis results when they are no longer useful. The
				1533	<tt>releaseMemory</tt> virtual method is the way to do this.</p>
				1534
				1535	<p>If you are writing an analysis or any other pass that retains a significant
				1536	amount of state (for use by another pass which "requires" your pass and uses the
				1537	<a href="#getAnalysis">getAnalysis</a> method) you should implement
				1538	<tt>releaseMEmory</tt> to, well, release the memory allocated to maintain this
				1539	internal state. This method is called after the <tt>run*</tt> method for the
				1540	class, before the next call of <tt>run*</tt> in your pass.</p>
				1541
				1542	</div>
				1543
				1544	<!-- *********************************************************************** -->
				1545	<div class="doc_section">
				1546	<a name="registering">Registering dynamically loaded passes</a>
				1547	</div>
				1548	<!-- *********************************************************************** -->
				1549
				1550	<div class="doc_text">
				1551
				1552	<p><i>Size matters</i> when constructing production quality tools using llvm,
				1553	both for the purposes of distribution, and for regulating the resident code size
				1554	when running on the target system. Therefore, it becomes desirable to
				1555	selectively use some passes, while omitting others and maintain the flexibility
				1556	to change configurations later on. You want to be able to do all this, and,
				1557	provide feedback to the user. This is where pass registration comes into
				1558	play.</p>
				1559
				1560	<p>The fundamental mechanisms for pass registration are the
				1561	<tt>MachinePassRegistry</tt> class and subclasses of
				1562	<tt>MachinePassRegistryNode</tt>.</p>
				1563
				1564	<p>An instance of <tt>MachinePassRegistry</tt> is used to maintain a list of
				1565	<tt>MachinePassRegistryNode</tt> objects. This instance maintains the list and
				1566	communicates additions and deletions to the command line interface.</p>
				1567
				1568	<p>An instance of <tt>MachinePassRegistryNode</tt> subclass is used to maintain
				1569	information provided about a particular pass. This information includes the
				1570	command line name, the command help string and the address of the function used
				1571	to create an instance of the pass. A global static constructor of one of these
				1572	instances <i>registers</i> with a corresponding <tt>MachinePassRegistry</tt>,
				1573	the static destructor <i>unregisters</i>. Thus a pass that is statically linked
				1574	in the tool will be registered at start up. A dynamically loaded pass will
				1575	register on load and unregister at unload.</p>
				1576
				1577	</div>
				1578
				1579	<!-- _______________________________________________________________________ -->
				1580	<div class="doc_subsection">
				1581	<a name="registering_existing">Using existing registries</a>
				1582	</div>
				1583
				1584	<div class="doc_text">
				1585
				1586	<p>There are predefined registries to track instruction scheduling
				1587	(<tt>RegisterScheduler</tt>) and register allocation (<tt>RegisterRegAlloc</tt>)
				1588	machine passes. Here we will describe how to <i>register</i> a register
				1589	allocator machine pass.</p>
				1590
				1591	<p>Implement your register allocator machine pass. In your register allocator
				1592	.cpp file add the following include;</p>
				1593
				1594	<div class="doc_code"><pre>
				1595	#include "llvm/CodeGen/RegAllocRegistry.h"
				1596	</pre></div>
				1597
				1598	<p>Also in your register allocator .cpp file, define a creator function in the
				1599	form; </p>
				1600
				1601	<div class="doc_code"><pre>
				1602	FunctionPass *createMyRegisterAllocator() {
				1603	return new MyRegisterAllocator();
				1604	}
				1605	</pre></div>
				1606
				1607	<p>Note that the signature of this function should match the type of
				1608	<tt>RegisterRegAlloc::FunctionPassCtor</tt>. In the same file add the
				1609	"installing" declaration, in the form;</p>
				1610
				1611	<div class="doc_code"><pre>
				1612	static RegisterRegAlloc myRegAlloc("myregalloc",
				1613	" my register allocator help string",
				1614	createMyRegisterAllocator);
				1615	</pre></div>
				1616
				1617	<p>Note the two spaces prior to the help string produces a tidy result on the
				1618	--help query.</p>
				1619
				1620	<div class="doc_code"><pre>
				1621	$ llc --help
				1622	...
				1623	-regalloc - Register allocator to use: (default = linearscan)
				1624	=linearscan - linear scan register allocator
				1625	=local - local register allocator
				1626	=simple - simple register allocator
				1627	=myregalloc - my register allocator help string
				1628	...
				1629	</pre></div>
				1630
				1631	<p>And that's it. The user is now free to use <tt>-regalloc=myregalloc</tt> as
				1632	an option. Registering instruction schedulers is similar except use the
				1633	<tt>RegisterScheduler</tt> class. Note that the
				1634	<tt>RegisterScheduler::FunctionPassCtor</tt> is significantly different from
				1635	<tt>RegisterRegAlloc::FunctionPassCtor</tt>.</p>
				1636
				1637	<p>To force the load/linking of your register allocator into the llc/lli tools,
				1638	add your creator function's global declaration to "Passes.h" and add a "pseudo"
				1639	call line to <tt>llvm/Codegen/LinkAllCodegenComponents.h</tt>.</p>
				1640
				1641	</div>
				1642
				1643
				1644	<!-- _______________________________________________________________________ -->
				1645	<div class="doc_subsection">
				1646	<a name="registering_new">Creating new registries</a>
				1647	</div>
				1648
				1649	<div class="doc_text">
				1650
				1651	<p>The easiest way to get started is to clone one of the existing registries; we
				1652	recommend <tt>llvm/CodeGen/RegAllocRegistry.h</tt>. The key things to modify
				1653	are the class name and the <tt>FunctionPassCtor</tt> type.</p>
				1654
				1655	<p>Then you need to declare the registry. Example: if your pass registry is
				1656	<tt>RegisterMyPasses</tt> then define;</p>
				1657
				1658	<div class="doc_code"><pre>
				1659	MachinePassRegistry RegisterMyPasses::Registry;
				1660	</pre></div>
				1661
				1662	<p>And finally, declare the command line option for your passes. Example:</p>
				1663
				1664	<div class="doc_code"><pre>
				1665	cl::opt<RegisterMyPasses::FunctionPassCtor, false,
Dan Gohman	8e58bc5	2008-10-14 17:00:38 +0000	[diff] [blame]	1666	RegisterPassParser<RegisterMyPasses> >
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1667	MyPassOpt("mypass",
				1668	cl::init(&createDefaultMyPass),
				1669	cl::desc("my pass option help"));
				1670	</pre></div>
				1671
				1672	<p>Here the command option is "mypass", with createDefaultMyPass as the default
				1673	creator.</p>
				1674
				1675	</div>
				1676
				1677	<!-- *********************************************************************** -->
				1678	<div class="doc_section">
				1679	<a name="debughints">Using GDB with dynamically loaded passes</a>
				1680	</div>
				1681	<!-- *********************************************************************** -->
				1682
				1683	<div class="doc_text">
				1684
				1685	<p>Unfortunately, using GDB with dynamically loaded passes is not as easy as it
				1686	should be. First of all, you can't set a breakpoint in a shared object that has
				1687	not been loaded yet, and second of all there are problems with inlined functions
				1688	in shared objects. Here are some suggestions to debugging your pass with
				1689	GDB.</p>
				1690
				1691	<p>For sake of discussion, I'm going to assume that you are debugging a
				1692	transformation invoked by <tt>opt</tt>, although nothing described here depends
				1693	on that.</p>
				1694
				1695	</div>
				1696
				1697	<!-- _______________________________________________________________________ -->
				1698	<div class="doc_subsubsection">
				1699	<a name="breakpoint">Setting a breakpoint in your pass</a>
				1700	</div>
				1701
				1702	<div class="doc_text">
				1703
				1704	<p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p>
				1705
				1706	<div class="doc_code"><pre>
				1707	$ <b>gdb opt</b>
				1708	GNU gdb 5.0
				1709	Copyright 2000 Free Software Foundation, Inc.
				1710	GDB is free software, covered by the GNU General Public License, and you are
				1711	welcome to change it and/or distribute copies of it under certain conditions.
				1712	Type "show copying" to see the conditions.
				1713	There is absolutely no warranty for GDB. Type "show warranty" for details.
				1714	This GDB was configured as "sparc-sun-solaris2.6"...
				1715	(gdb)
				1716	</pre></div>
				1717
				1718	<p>Note that <tt>opt</tt> has a lot of debugging information in it, so it takes
				1719	time to load. Be patient. Since we cannot set a breakpoint in our pass yet
				1720	(the shared object isn't loaded until runtime), we must execute the process, and
				1721	have it stop before it invokes our pass, but after it has loaded the shared
				1722	object. The most foolproof way of doing this is to set a breakpoint in
				1723	<tt>PassManager::run</tt> and then run the process with the arguments you
				1724	want:</p>
				1725
				1726	<div class="doc_code"><pre>
				1727	(gdb) <b>break llvm::PassManager::run</b>
				1728	Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
				1729	(gdb) <b>run test.bc -load $(LLVMTOP)/llvm/Debug/lib/[libname].so -[passoption]</b>
				1730	Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug/lib/[libname].so -[passoption]
				1731	Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
				1732	70 bool PassManager::run(Module &M) { return PM->run(M); }
				1733	(gdb)
				1734	</pre></div>
				1735
				1736	<p>Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are
				1737	now free to set breakpoints in your pass so that you can trace through execution
				1738	or do other standard debugging stuff.</p>
				1739
				1740	</div>
				1741
				1742	<!-- _______________________________________________________________________ -->
				1743	<div class="doc_subsubsection">
				1744	<a name="debugmisc">Miscellaneous Problems</a>
				1745	</div>
				1746
				1747	<div class="doc_text">
				1748
				1749	<p>Once you have the basics down, there are a couple of problems that GDB has,
				1750	some with solutions, some without.</p>
				1751
				1752	<ul>
				1753	<li>Inline functions have bogus stack information. In general, GDB does a
				1754	pretty good job getting stack traces and stepping through inline functions.
				1755	When a pass is dynamically loaded however, it somehow completely loses this
				1756	capability. The only solution I know of is to de-inline a function (move it
				1757	from the body of a class to a .cpp file).</li>
				1758
				1759	<li>Restarting the program breaks breakpoints. After following the information
				1760	above, you have succeeded in getting some breakpoints planted in your pass. Nex
				1761	thing you know, you restart the program (i.e., you type '<tt>run</tt>' again),
				1762	and you start getting errors about breakpoints being unsettable. The only way I
				1763	have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are
				1764	already set in your pass, run the program, and re-set the breakpoints once
				1765	execution stops in <tt>PassManager::run</tt>.</li>
				1766
				1767	</ul>
				1768
				1769	<p>Hopefully these tips will help with common case debugging situations. If
				1770	you'd like to contribute some tips of your own, just contact <a
				1771	href="mailto:sabre@nondot.org">Chris</a>.</p>
				1772
				1773	</div>
				1774
				1775	<!-- *********************************************************************** -->
				1776	<div class="doc_section">
				1777	<a name="future">Future extensions planned</a>
				1778	</div>
				1779	<!-- *********************************************************************** -->
				1780
				1781	<div class="doc_text">
				1782
				1783	<p>Although the LLVM Pass Infrastructure is very capable as it stands, and does
				1784	some nifty stuff, there are things we'd like to add in the future. Here is
				1785	where we are going:</p>
				1786
				1787	</div>
				1788
				1789	<!-- _______________________________________________________________________ -->
				1790	<div class="doc_subsubsection">
				1791	<a name="SMP">Multithreaded LLVM</a>
				1792	</div>
				1793
				1794	<div class="doc_text">
				1795
				1796	<p>Multiple CPU machines are becoming more common and compilation can never be
				1797	fast enough: obviously we should allow for a multithreaded compiler. Because of
				1798	the semantics defined for passes above (specifically they cannot maintain state
				1799	across invocations of their <tt>run*</tt> methods), a nice clean way to
				1800	implement a multithreaded compiler would be for the <tt>PassManager</tt> class
				1801	to create multiple instances of each pass object, and allow the separate
				1802	instances to be hacking on different parts of the program at the same time.</p>
				1803
				1804	<p>This implementation would prevent each of the passes from having to implement
				1805	multithreaded constructs, requiring only the LLVM core to have locking in a few
				1806	places (for global resources). Although this is a simple extension, we simply
				1807	haven't had time (or multiprocessor machines, thus a reason) to implement this.
				1808	Despite that, we have kept the LLVM passes SMP ready, and you should too.</p>
				1809
				1810	</div>
				1811
				1812	<!-- *********************************************************************** -->
				1813	<hr>
				1814	<address>
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				1819
				1820	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
				1821	<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
				1822	Last modified: $Date$
				1823	</address>
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